Protein K is the only major outer membrane protein found in each of 33 encapsulated pathogenic E. coli strains. As such, it is an excellent candidate for development of a protective vaccine. Protein K has been shown to function in vivo and in vitro as a porin. The normal expression of the K1 polysaccharide capsule requires a functional porin. This may reflect the role of protein K in the outer membrane of pathogenic E. coli K1 strains. The synthesis of protein K in E. coli K-12 strains is regulated by the envZ gene in a manner different than the regulation of the OmpF and OmpC porins.